1. Field of the invention
The present invention relates, in general, to a process for the preparation of semi-permeable membrane superior in selectivity and permeability and, more particularly, to corona treatment of a reinforcement for semi-permeable membrane, thereby improving bonding strength between a porous support layer and the reinforcement and mechanical strength of semi-permeable membrane.
2. Description of the Prior Art
It is known that various dissolved substances or solutes can be separated from their liquids or solvents by various useful separation process or techniques known as ultrafiltration or reverse osmosis. Especially, reverse osmosis membranes are high permeability barriers. For example, In a liquid-liquid system, reverse osmosis membranes have high permeability for water yet are impermeable to microorganisms, colloidal particles, salts and organic materials, so that these solutes or undesirable materials can be removed.
Reverse osmosis technology, which is generally applied to separation of solutes with a size of 10 Angstrom or less, is efficient for the desalination of seawater, contaminated water or saline, to obtain pure water for drink or other uses. That is, the use of this technology which has apparently gained the widest attention to date is the desalination of brackish water or seawater to provide large volumes of relatively non-salty water for industrial, agricultural, or home use.
Purification of water through reverse osmosis membrane is accomplished by applying pressure to the seawater or contaminated water to force the water through the reverse osmosis membrane, so that purified water passes through the membrane and the salt or contaminant is rejected.
With regard to the characteristics of reverse osmosis membrane, first, it should have high salt rejection coefficient. For commercial practicality, it is required salt rejection capabilities of 98% or more. In addition, the reverse membrane should also permit high fluxes at reasonable pressures in a view of economy. Several additional requirements may be imposed on the commercial applications of reverse osmosis membrane, according to the realities of such applications. Among the additional requirements, chlorine resistance is necessary for the reverse osmosis membrane to be used for, especially, tap water.
Separation technologies using separation films are virtually based on two factors: pressure and concentration. For example, precise filtration or ultrafiltration is dependent on the former factor, whereas gas separation or permeable evaporation is on the latter factor. Reverse osmosis is based on both pressure and concentration and reverse osmosis membranes, which can filter monovalent ions or salts incapable of being filtered by precise filter membrane or ultrafiltration membrane, are useful for a wide range of uses including desalination of seawater, treatment of bittern, treatment of industrial waste water, preparation of ultra deionized water for washing semiconductor device.
In order to allow for high fluxes in desalination, the optimum type of reverse osmosis membrane is extremely thin. However, extremely thin membranes lack structural integrity as they stands. In many cases, the reverse osmosis membrane is synthesized or laminated on a porous support layer reinforcement. Of cause, the porous support layer reinforcement is required to have pore sizes large enough to pass the permeated water through the porous support layer without reducing or affecting the fluxes.
However, large pores of the porous support layer reinforcement may provide the thin synthetic semi-permeable membrane with an opportunity of filling or infiltrating the pores, so that there would be potent possibility of membrane breakdown upon operation under high pressure or of membrane deformation. Accordingly, large size of the pores on the porous support layer reinforcement is not always good, since it is apt to lose the reverse osmosis capability of the thin synthetic semi-permeable membrane as just mentioned.
Reverse osmosis membranes have been prepared from a wide variety of known polymeric materials. Many of the reverse osmosis membranes posses salt rejection capabilities of 98% or more. A part of the reverse osmosis membranes cannot permit satisfactory amount of water per area of membrane, e.g. flow rate or flux, which is another important factor for commercial practicality.
Prior patents have suggested various membranes useful for reverse osmosis technology. For example, U.S. Pat. No. 4,277,344 describes a semi-permeable membrane prepared from a crosslinked aromatic polyamine which is a product of interfacial polymerization between monomeric polyacyl halide and monomeric arylene polyamine.
U.S. Pat. No. 4,415,445 discloses a membrane useful for desalination of water prepared from a copolymer consisting of a monomer containing hydroxyl group, such as methacrylic acid and hydroxyethyl methacrylic acid, and optionally additional multifunctional carboxylic acid, such as 1,2,4,5-bezenetetracarboxylic acid or citric acid.
U.S. Pat. No. 4,168,352 teaches a process for producing a membrane useful for reverse osmosis, using a monomer of hydroxyethylmethacrylate and another polymer, such as polyacrylic acid, closely mixed together. In this patent, polymerization of hydroxyethylmethacylate is conducted in a photochemical manner, and the resulting membrane comprises a homopolymer of hydroxyethylmethacrylate mixed with different polymers. There is no chemical bond between the polymerized hydroxyethylmethacrylate and polycarboxylic acid.
U.S. Pat. No. 4,267,295 describes hydrogel which is prepared by crosslinking acrylates, such as glycerlymethacrylate, methylmethacrylate and hydroxyethylacrylate, with diacrylate, such as ethyleneglycoldimethacrylate. Polymerization for the hydrogel is initiated by a free radical initiator, such as azobisisobutyronitrile. The resulting film has free hydroxyl groups which are unreacted during formation of the film and is composed of crosslinked, heterogeneous polymer of various monomers.
In addition, there are many patents that are concerned with composite semi-permeable membranes using various materials. Among the patents are U.S. Pat. Nos. 3,744,642, 4,005,012, 4,366,062, 4,606,943, 4,618,534, 4,634,531, 4,661,254, 4,761,234, and 4,783,346.
To date, interfacially synthesized reverse osmosis membranes which are described in U.S. Pat. No. 4,277,344 have generally been adopted, in consideration of general aspects of membrane capability.
As previously mentioned, the porous support layer for composite semi-permeable membrane must have mechanical strength and toughness sufficient to endure under high pressures as well as pore sizes large enough not to be resistant to water flow. In addition, it should satisfy a requirement that a porous support layer solution is not transuded from one surface of the reinforcement when it is cast on the other surface thereof. For example, if the porous support layer solution infiltrates into and is transuded to the opposite surface of reinforcement, the membrane is adversely affected in an aspect of the flux, although not in selective permeability. This ill influence gives rise to increasing difficulty in operating process steps, continuously and thus, in large scale.
In an effort to solve the above problems, U.S. Pat. No. 3,912,834 intended to improve the mechanical strength and flux of semi-permeable membrane, along with a production process therefor. In this process, the degree of infiltration of a support layer substance into a support layer reinforcement is controlled by soaking a back side of a porous fabric, the support layer reinforcement, in a solvent incompatible with the support layer substance and casting the support layer substance on a front side of the porous fabric. However, this prior technology neither is operated continuously nor exhibits satisfactory flux.